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« 


H  Ferric  Oxide  Hydrosol 


Submitted  in   Partial  Fulfillment  of  the  Requirements   for  the 

Degree  of  Doctor  of  Philosophy  in  the  Faculty  of 

Pure   Science,   Columbia  University. 


By 


ALEXANDER  FRIEDEN,  B.S.,M.S..M.A. 


NEW  YORK  CITY 
1922 


Ferric  Oxide  Hydrosol 


Submitted  in  Partial  Fulfillment  of  the  Requirements  for  the 

Degree  of  Doctor  of  Philosophy  in  the  Faculty  of 

Pure  Science,  Columbia  University. 


By 
ALEXANDER  FRIEDEN,  B.S.,M.S.,M.A. 


NEW  YORK  CITY 
1922 


ACKNOWLEDGMENT 

The  author  wishes  to  express  his  sincere  appreciation  for  the 
constant  advice  and  guidance  of  Professor  A.  W.  Thomas,  at 
whose  suggestion  this  research  was  undertaken. 

KXCMAMCHC 


HISTORICAL 

The  history  of  ferric  oxide  hydrosol  may  be  divided  into 
three  periods:  (1)  1827  to  1855,  (2)  1855  to  1909,  and  (3)  1909 
to  the  present  time. 

The  reports  by  Berzelius,1  Maus,2  Souberain,3  Rose,4  Schon- 
bein,5  and  Schorer,6  during  the  first  period,  showed  that  they 
regarded  colloidal  ferric  oxide  as  a  definite  chemical  compound 
of  iron. 

The  second  period  starts  with  the  discovery  of  Pean  de  St. 
Gilles7  that  when  ferric  acetate  solution  was  heated  on  a  water 
bath  for  four  or  five  hours,  the  red  solution  became  turbid  by 
reflected,  while  clear  to  transmitted  light.  A  trace  of  alkali  or 
sulfuric  acid  precipitated  all  of  it  in  the  form  of  a  red  brown 
residue,  insoluble  in  concentrated,  but  soluble  in  dilute  acid.  The 
acid  precipitate  when  dried  on  a  porus  plate,  turned  dark  brown. 
In  this  condition  it  was  still  readily  soluble  in  water  but  it  lost 
this  property  after  being  thoroughly  dried. 

Scherer  Kastner8  prepared  a  similar  colloidal  ferric  oxide 
by  plunging  test  tubes  containing  dilute  solutions  of  ferric  nitrate 
into  boiling  water,  and  Debray9  obtained  a  similar  hydrosol  upon 
heating  dilute  solutions  of  ferric  chloride.  Wiedemann10  showed 
that  any  solution  of  a  ferric  salt  contains  some  colloidal  oxide  in 
it ;  and  recently  Wagner11  showed  that  a  1-400  equivalent  normal 
solution  of  ferric  chloride  hydrolyzes  completely  to  colloidal 
ferric  oxide  and  hydrochloric  acid  in  40  minutes  at  25°. 

Collodial  ferric  oxide  was  regarded  by  these  early  scientific 
workers  as  a  modification  of  ferric  hydroxide,  but  all  the  early 
attempts  to  determine  the  nature  and  composition  of  the  sub- 
stance were  doomed  to  failure  because  in  all  cases  the  substance 
formed  contained  not  only  the  products  but  all  the  reagents  in- 
volved, as  well.  This  difficulty  was  removed  by  the  introduction 
of  the  process  of  dialysis  by  Thomas  Graham.12 

Thomas  Graham  prepared  his  ferric  oxide  "solution"  by 
disolving  freshly  precipitated  ferric  hydroxide  in  a  solution  of 
ferric  chloride.  This  solution,  containing)  from  4  to  5  percent 
of  solid  matter,  was  dialyzed  through  a  parchment  membrane 
for  11  days  against  distilled  water.  Analysis  of  it  showed  that 
the  sol  consisted  of  one  equivalent  of  hydrochloric  acid  to  30.J) 
equivalents  of  ferric  oxide,  the  hydrochloric  acid  being  regarded 
as  an  impurity  by  Graham.  (In  this  connection  it  is  interesting 
to  note  that  Berzelius1  prepared  colloidal  ferric  oxide  by  the 
method  that  Graham  used,  but  called  it  "basic  ferric  chloride".) 

3 


That  an  iron  oxide  hydrosol  prepared  from  ferric  chloride  con- 
tains chloride  was  also  shown  by  Magnier  de  la  Source13  and 
by  Wyrouboff.14 

Hantz  and  Desch15  dialyzed  Graham's  sol  to  the  absence  of 
chloride  iron  as  tested  by  the  addition  of  silver  nitrate  to  the  so! 
and  then,  upon  quantitative  analysis  of  the  sol  found  that  it  con- 
tained a  great  deal  of  chloride.  The  analysis  showed  that  there 
was  one  molecule  of  ferric  chloride  to  -every  18  molecules  of  ferric 
hydroxide.  They  explained  the  failure  to  get  the  qualitative  test 
for  chloride  ion  as  due  to  the  formation  of  a  "chlorine  complex". 
Ruer16  made  the  same  observations,  but  he  attributed  the  failure 
of  silver  nitrate  to  show  the  presence  of  chloride  to  a  "protective" 
action  of  the  sol,  an  "explanation"  which  unfortunately  is  fre- 
quently used  in  colloidal  chemistry. 

Linder  and  Picton17  obtained  results  which  agreed  with  those 
of  Hantz  and  D-esch  and  concluded  that,  "since  on  dialysis  of  a 
solution  of  ferric  hydroxide  and  ferric  chloride  for  sixty-one 
days,  hydrochloric  acid  could  still  be  detected;  and  on  another 
occasion,  decided  traces  of  chlorine  could  be  detected  in  the 
dialysis  product  its-elf  after  210  days,  the  substance  present  here 
is  a  hydroxychloride  and  not  a  hydrate  associated  with  ferric 
chloride  or  free  hydrochloric  acid." 

Studying  the  composition  of  the  substances  formed  by  heat- 
ing dilute  solutions  of  ferric  chloride,  Krecke18  concluded  that 
dep-ending  on  the  concentration  of  the  salt  and  on  the  temperature, 
the  solution  would  undergo  the  following  changes :  first,  the 
iron  oxide  of  Graham  will  be  formed,  then  that  of  Pean  de  St. 
Gilles,  then  the  oxychloride,  and  finally,  ordinary  ferric  oxide. 

The  third  period  in  the  history  of  ferric  oxide  hydrosol  began 
with  the  enunciation  of  the  "Complex  Theory"  of  colloids  by 
P.  P.  von  Weimarn.19  He  and  others  applied  only  to  the  common 
metals  but  it  has  been  recently  shown  by  Beans  and  Eastlack20 
that  it  applies  to  the  nobk  metals  as  well.  In  connection  with 
ferric  oxide  hydrosol,  this  idea  was  first  taken  up  by  Malfitano.21 
Reasoning  from  the  freezing  point  depression,  he  concluded  that 
because  of  the  low  depression — much  lower  than  would  be 
observed  if  the  chloride  were  in  the  ionic  condition — the  complex 
must  exist  in. the  form  somewhat  as  follows: 

[(FeO3H3)n    FeCl2]Cl,    [(FeO3H3)nFeCl]Cl2, 
{  [(FeO3H3)nFeO2H2Cl]mFe  }  C13 

Neidle22  attempted  to  determine  the  purity,  or,  rather,  the 
amount  of  electrolyte  present  in  a  sol  by  obtaining  a  number  of 
precipitation  values  with  sulfuric  acid,  and,  plotting,  these  values 
against  the  hydrochloric  acid  contents  as  determined  analytically, 
he  extrapolated  the  maximum  purity  of  that  particular  sol.  The 
assumptions  were  made1  that  an  equivalent  of  •'sulfuric  acid 
displaces  an  equivalent  of  hydrochloric  and  that  this  is  con- 


tinued  on  extrapolation.  He  concluded  from  such  results  that 
there  is  a  series  of  "oxychlorides,"  that  when  a  colloid  is 
dialyzed  to  a  certain  point,  a  gel  will  be  formed  and  a  sol  of  less 
purity  will  remain.  This  will,  on  further  dialysis,  precipitate 
another  gel  with  the  formation  of  another  sol,  etc.,  thus  under- 
going a  series  of  changes.  He  considers  that  the  first  colloid  has 
a  ratio  of  21  Fe-3  to  Cl,  and  cites  as  further  evidence  the  fact 
that  a  solution  of  ferrous  sulfate  oxidized  in  air  in  presence  of 
metallic  platinum  forms  a  precipitate  at  about  that  ratio. 

The  idea  of  the  formation  of  a  chain  of  compounds  is  not 
new.  Von  Weimarn23  uses  a  stepladder  as  illustration  of  the 
behavior  of  aluminum  chloride  upon  dialysis.  Aluminum  chloride 
is  at  the  highest  rung  and  aluminum  hydroxide  at  the  lowest,  each 
rung  from  top  downward  representing  a  different  substance  con- 
taining less  free  energy  and  being  less  soluble  than  the  preceding 
one. 

Neidle  disagrees  with  Nicolardot24  who  believes  that  there 
are  two  oxychlorides,  one  having  the  ratio  in  equivalents  of 
6  Fe  to  1  Cl,  the  other  of  125  to  1.  The  first  no  longer  gives  a 
test  for  ferric  ion,  the  second  shows  absence  of  chloride  ion. 
According  to  Nicolardot,  any  particular  sol  is  composed  of  a 
mixture  of  these  two  oxychlorides. 

Pauli  and  Matula25  consider  ferric  oxide  hydrosol  to  be 
composed  of  a  complex  according  to  the  Complex  Theory,  and 
picture  it  as  xFe(OH)3yFe+++3yCl — .  They  found  their  prep- 
•arations  to  be  neutral  in  reaction  and  to  have  a  chloride  ion 
concentration  less  than  the  total  chlorine  present ;  also  that  the 
concentration  of  the  chloride  ion  increases  with  dilution. 


CRITICISM 

The  question  of  the  nature  of  ferric  oxide  hydrosol  is  far 
from  settled.  Nicolardot's  assertion  that  there  are  two  oxy- 
chlorides, one  when  the  test  for  ferric  ion  is  absent  and  the 
other  on  the  absence  of  chloride  ion,  is  questionable  since  the 
test  for  ferric  ion  by  ammonium  thiocyanate  is  shown  at  much 
higher  purities  than  those  recorded  by  him. 

Magnier  de  la  Source13  stated  that  the  sols  which  he  analyzed 
were  perfectly  clear  and  "gave  the  ferric  test  with  potassium 
ferrocyanide."  Well  dialyzed  sols,  we  found,  do  not  give  this 
test,  precipitation  of  ferric  oxide  gel  taking  place  without  evi- 
dence of  blue  color.  Magnier  de  la  Source  could  not  have  had 
sols  of  highest  purity. 

The  efforts  of  Linder  and  Picton  were  valuable  in  that  they 
emphasized  the  fact  that  chloride  "impurity"  was  an  essential 
part  of  the  sol,  but  their  quantitative  data  concerning  the  compo- 

5 


sition  of  the  sols  are  without  value  since  they  could  not  have  ob- 
tained more  than  a  fraction  of  the  chloride  present  by  the  pro- 
cedure which  they  employed.  The  earlier  work  of  Krecke  and 
Pean  de  St.  Gilles  show  how  faulty  Malfitano's  method  was,  and 
therefore  his  results  are  not  conclusive. 

Neidle  considered  his  dialysis  complete  when  addition  of 
ammonium  thiocyanate  to  the  sol  gave  no  color,  after  the  precipi- 
tate settled  or  was  centrifuged.  Since  ferric  oxide  gel  carries 
down  ferric  chloride  with  it,  his  failure  to  obtain  the  color 
reaction  does  not  mean  that  all  the  ferric  ion  in  excess  of  that 
absorbed  had  been  dialyzed  away.  Furthermore,  in  his  deter- 
mination of  chloride,  the  sol  was  heated  after  concentrated  nitric 
acid  had  been  added  to  it,  to  accomplish  solution  of  the  gel.  We 
have  found  that  it  requires  a  high  temperature  and  prolonged 
heating  to  effect  this  and  hence  there  is  the  danger  of  loss  of 
chlorine.  His  precipitation  values  are  entirely  relative.  An 
amount  of  sulfuric  acid  which  will  not  completely  precipitate  a 
sol  in,  say,  five  minutes,  will  do  so  upon  longer  standing.  It  is 
doubtful  if  the  method  of  extrapolation  is  valid. 


METHODS 

Preparation  of  the  Hydro  sols 

The  following  methods  were  used  to  prepare  ferric  oxide 
hydrosols : 

(1)  About    14    M    ammonium    hydroxide    solution    was 
delivered  drop  by  drop  from  a  burette  into  a  solution  of  3  M 
ferric  chloride,  which  was  continually  and  vigorously  agitated 
by  a  motor  stirrer.    The  addition  of  ammonium  hydroxide  solu- 
tion was  continued  until  the  resultant  precipitate  no  longer  readily 
dispersed. 

(2)  Ammonium   hydroxide   was    added   as   above   until   a 
permanent  precipitate  was  just  formed.      l/z   M   ferric  chloride 
solution  was  then  added,  and  the  mixture  stirred  until  The  precipi- 
tate dispersed. 

(3)  To  a  hydrosol  prepared  by  method  (1)   ferric  chloride 
solution  was  added  until  the  entire  sol  precipitated,  dispersing  the 
resulting  precipitate  in  distilled  water. 

(4)  50  cc.  of  molar  hydrochloric  acid  were  added  to  a  freshly 
prepared  and  washed  precipitate  of   ferric  hydroxide  prepared 
'from  250  cc.  3  M  FeCl3.    The  mixture  was  allowed  to  stand  until 
the  precipitate  was  peptized. 

The  hydrosols  prepared  by  the  first  two  methods  were  blood 
red  in  color  and  perfectly  clear  to  reflected  and  transmitted  light. 
Those  prepared  by  the  third  method  were  clear  to  transmitted, 


but  slightly  turbid  to  reflected  light,  while  the  sols  prepared  by 
the  last  method  were  decidedly  turbid  to  reflected,  though  clear 
to  transmitted  light. 

Dialysis 

Cups*  of  very  fine  unglazed  porcelain  were  first  tried  as 
dialyzers.  It  was  found,  however,  that  these  were  permeable  to 
ferric  ions  for  only  a  short  time.  After  a  few  days  of  dialysis, 
there  was  but  a  slight  amount  of  ferric  ion  in  the  diffusate, 
though  the  hydrosol  in  the  cup  contained  a  large  amount  of  ferric 
chloride.  Evidently,  the  membrane  of  ferric  oxide  formed  within 
the  walls  of  the  cup  is  impermeable  to  ferric  ion. 

Unglazed  porcelain  cups  could  not,  therefore,  be  used  at  this 
stage  of  dialysis.  In  subsequent  dialysis  experiments,  where 
all  of  the  unadsorbed  ferric  chloride  had  already  left  the  solution, 
and  further  dialysis  was  merely  a  slow  hydrolysis,  porcelain  cups 
of  smaller  size  and  thinner  walls  were  employed.  These  were 
permeable  to  chloride  and  hydrogen  ions. 

For  the  preliminary  dialysis,  collodion  bags  were  -employed. 
These  bags  were  prepared  in  a  two  liter  Florence  flask  and  were 
changed  as  soon  as  a  coating  of  ferric  oxide  was  formed  on  the 
walls  of  the  membrane,  in  order  to  speed  up  the  process  of 
dialysis.  Some  of  these  sols  were  dialyzed  at  room  temperature 
and  others  at  60°.  The  diffusate  was  changed  every  twenty-four 
hours,  distilled  water  being  used  throughout.  The  preliminary 
dialysis  was  considered  complete  when  the  diffusate  of  twenty- 
four  hours,  usually  one  liter  in  volume,  acidified  and  evaporated 
to  10  cc.,  gave  no  color  upon  addition  of  5  cc.  of  molar  ammonium 
thiocyanate.  Since  this  is  a  very  delicate  test  for  the  ferric  ion, 
its  concentration  in  the  hydrosol  is  negligible  at  this  point.  (The 
ratio  of  the  total  ferric  oxide  in  grams  to  the  total  ferric  chloride 
in  grams — F^Os/FeCls — was  about  10  at  this  point.)  The  test 
for  the  ferric  ion  as  given  by  ammonium  thiocyanate  applied  to 
the  sol  directly  was  negative  long  before  this  ratio  was  reached. 

The  length  of  time  required  for  the  completion  of  dialysis 
varies  from  two  to  five  months,  depending  upon  the  concentration 
of  the  sol  and  the  temperature  at  which  the  dialysis  is  performed ; 
the  more  dilute  the  sol  is,  the  quicker  is  the  hydrolysis  process, 
and  the  easier  for  the  dialyzable  hydrolytic  substances  to  leave 
the  solution.  Dilution  of  the  dialyzing  hydrosol  was  avoided  by 
having  the  level  of  the  water  in  the  outer  vessel  much  lower  than 
that  of  the  hydrosol  inside. 

ANALYSES 

Iron. 

Iron  was  determined  by  titration  with  potassium  perman- 

*   Obtained  from   Coors  Porcelain   Company,   Golden,   Colorado 

7 


ganate,  the  colloidal  solution  having  been  evaporated  with  sul- 
furic  acid  to  fumes  of  sulfur  trioxide  and  reduced  by  the  Jones 
reductor. 

Chlorine. 

Since  hydrogen  ions  peptize  the  colloidal  particks,  due  to 
their  combination  with  hydroxyl  ions  that  may  be  associated  with 
the  complexes  as  a  result  of  the  hydrolysis  of  the  absorbed  ferric 
chloride,  nitric  acid  dissolves  the  sol  with  great  dificulty.  The 
first  effect  of  the  acid  is  to  make  the  hydrosol  more  stable,  but  on 
the  addition  of  a  considerable  amount,  precipitation  of  hydrous 
ferric  oxide  takes  place.  This  precipitate  redissolves  only  on 
prolonged  contact  with  the  acid. 

This  caused  unexpected  difficulties  in  the  analyses  of  the 
sols  for  the  determination  of  chlorine. 

Aifter  considerable  experimentation,  the  following  method 
was  adopted  for  the  dissolution  of  the  precipitated  hydrosol  and 
determination  of  the  chloride  concentration, — 

To  a  definite  volume  of  the  hydrosol,  nitric  acid  was  added 
to  make  the  final  concentration  about  three  molar.  The  covered 
beaker  wras  allowed  to  stand  in  the  dark  until  all  the  ferric  oxide 
had  dissolved,  which  usually  required  a  week  or  ten  days.  Ap- 
proximately 0.1  molar  silver  nitrate  was  then  added  in  excess  of 
the  amount  necessary  to  precipitate  all  the  chloride.  A  slight 
variation  in  the  procedure  was  to  add  the  measured  amount  of 
silver  nitrate  before  the  addition  of  nitric  acid.  Separate  ex- 
periments, however,  on  pure-,  potassium  chloride  showed  that 
the  procedures  could  be  used  ^iterchangeably,  and  that  there  was 
no  danger  of  loss  of  chloride"  by  oxidation.  The  chloride  con- 
centration was  determined  either  gravimetrically  or  by  the  method 
of  Volhard. 

EXPERIMENTAL 

At  the  beginning  of  this  investigation,  measurements  of  the 
conductivity  of  ferric  oxide  hydrosol  during  dialysis  were  made 
with  the  hope  of  getting  quantitative  indication  of  an  end  point 
in  the  purification.  The  measurements  showed,  as  anticipated, 
that  the  conductivity  gradually  decreases  as  dialysis  proceeds,  but 
after  a  certain  time  the  hydrosol  showed  a  conductivity  lower 
than  that  of  the  distilled  water  against  which  it  was  being 
dialyzed.  The  conductivity  of  ferric  oxkle  hydrosol  has  been 
reported  by  several  investigators  20,  23,  2G,  but  the  results  differ 
widely  due  to  the  variable  quantities  of  peptizing  electrolyte 
present  and  are  obviously  of  no  value. 

Due  to  the  failure  of  the  conductivity  method,  freezing  point 
depression  was  tried,  and  since  this  also  failed  to  serve  our  pur- 


pose  (see  later),  the  observation  of  beginning  of  precipitation  was 
adopted  as  end  point  in  dialysis.  The  incipience  of  precipitation 
does  not  by  any  means  signify  the  end  of  the  hydrosol,  since  there- 
after precipitation  proceeds  gradually,  the  system  assuming  a 
turbid  appearance  which  increases  with  continued  dialysis  until 
finally  the  entire  sol  becomes  a  gel. 

The  ammonium  salts  formed  in  the  preparation  of  the  sol 
disappear  within  a  comparatively  short  time  after  the  beginning 
of  dialysis.  Thus,  for  sol  No.  9,  no  traces  of  ammonium  salts 
were  detectable  after  two  weeks  dialysis.  When  dialyzed  for 
four  weeks,  this  sol  contained:  Fe2O3, —  4.6695  gm./L*  and 
FeCl3,  —0.2664  gm./L*  showing  a  molar  ratio  Fe2O3:FeCl3 
=11.7. 

A  part  of  this  hydrosol  was  removed  from  the  large  col- 
lodion bag  and  placed  in  a  small  bag  in  order  that  the  incipience 
of  precipitation  might  be  obs-erved  more  easily.  The  level  of  the 
water  outside  was  kept  lower  than  that  of  the  sol  inside  the  sack 
in  order  to  present  dilution.  Upon  the  first  appearance  of  a 
precipitate,  analysis  of  the  hydrosol  showed :  Fe2O3,  — 7.0713 
gm./L,  FeCl3,  —0.3282  gm./\L  giving  a  molar  Fe2O3:  FeCl3 
=21.8. 

Two  and  one  half  liters  of  sol  No.  10  were  dialyzed  for  four 
weeks  at  room  temperature  against  a  volume  of  one  liter  of 
distilled  water,  the  outside  water  being  changed  several  times 
each  day  for  the  first  week  and  once  a  day  thereafter.  Samples 
were  withdrawn  for  analysis  at  intervals  as  noted  in  Table  I 
until  precipitation  began,  after  which  the  analyses  were  made 
less  frequently,  and  dialysis  was  continued  until  almost  all  of 
the  sol  was  converted  to  a  gel.  This  took  place  after  ten  weeks  of 
continued  dialysis. 

TABLE  I 


Number 
of 
Sample 

Time    of            Ferric    Oxide 
Dialysis               (Fe2O3) 

Molar 
7&  r?,londe        Ratio    of 
(FeCls)               Fe203/FeCl3 

lOa 

24  days            9.310&  gm/L 

0.7143  gm./L             13.2 

lOb 

27 

4.2923 

0.2754     ' 

15.9 

lOc 

32 

3.6443 

0.2150     ' 

17.7 

lOd 

40 

3.3555 

0.1720     ' 

20.3 

lOe(ppfion) 

47 

3.2653 

a.  1539     ' 

21.5 

lOf 

52 

3.4588 

0.1080     ' 

23.6 

10g 

60 

*•               1.6704 

0.06039  ' 

28.1 

10h*(finaJ) 

73     '                  1.4347 

0.04965  '                        29.3 

Precipitate 

47.9 

*  In  all  tabulations  throughout  this  paper,  the  chlorine  found  by  analysis  was  calculated 
to   ferric  chloride,    the   ferric   chl  .ride   computed   to   ferric   oxide   which    was   subtracted 
from  the  total  ferric  oxide  (from  total  iron  found  by  analysis),  giving  the  ferric  oxide 
values   us?d   in   the   data. 

*  In  the  interval  between    lOa  and   lOb,  the  sol  became  considerably  more  dilute  due 
to    an    accident    in    the    collodion    bag.      Precipitation    began    after    47    days    of    dial- 
ysis.  lOh  was  almost  entirely  in  the  gel  state.   It  was  dried  in  an  oven  for  six  hours  at 
110  deg,  for  three  hours  at  160  deg.,  powdered,  washed  until  several  consecutive  wa=,r- 
ings  gave  the  same  turbidity  with  silver  nitrate  when  viewed  in  test  tubes,  and  analyzed. 
The   results   of   the   analysi's   are   shown   under   the   heading   "precipitate"    in    the   above 
table. 

9 


Sol  No.  15  was  dialyzed  at  60°  for  ten  weeks.  At  the  end 
of  this  time,  the  twenty-four  hour  diffusate  when  evaporated  to 
10  cc.  volume  gave  no  test  for  ferric  ion.  Five  hundred  cc.  of 
this  sol  were  diluted  to  about  3  liters  (No.  15a)  and  a  series  of 
five  collodion  bags  of  500  cc.  each  was  allowed  to  dialyze. 

TABLE  II 


Number 
of 
Sample 

Time    of 
Dialysis 

Ferric    Oxide 
(Fe203) 

Ferric     Chloride 
(FeCl3) 

Molar 
Ratio    of 
Fe203/FeCl3 

15 
15a 
I5b 
15c 
15d 
ISe 
1 

18  days 
21     " 
30     " 
40     ' 

81017  gm./L 
6.1521     ' 
1.1961     " 
1.1960     " 
1.4164     " 
1.5773     ' 

0.8397  gm./L 
0.3330     ' 
0.05927  " 
0.05924  " 
0.05552  " 
0.04896  " 

9.8 
'18.9 
20.5 
20.5 
25.8 
32.7 

No.  15  represents  the  analysis  of  the  original  impure  sol  at 
the  time  the  preliminary  dialysis  was  stopped.  No.  15a  represents 
the  original  sol  dialyzed  until  precipitation  began ;  it  became 
somewhat  diluted  during  this  process.  The  diluted  sols  showed 
a  slight  precipitation  after  18  days  of  dialysis,  but  no  turbidity 
could  be  detected  in  the  supernantant  solution.  However,  with 
continued  dialysis,  precipitation  increased,  and  after  forty  days 
of  dialysis  a  large  part  of  the  sol  in  15e  had  turned  to  a  gel. 

This  would  seem  to  indicate  that  dilution  has  no  marked 
effect  on  the  point  at  which  precipitation  begins.  To  verify  this, 
sol  No.  13  which  had  been  dialyzed  at  60°  for  8  weeks,  was  mad-e 
up  to  various  dilutions  and  these  allowed  to  dialyze  in  500  cc. 
collodion  bags  to  first  appearance  of  precipitate : 

TABLE  III 


Number 
of 
Sample 

Dilution 

Ferric  Oxide      Ferric   Chloride 
(Fe2O8)                    (Fed,) 

Molar 
Ratio  of 
Fe203/FeCl3 

13 

4.1838  gm./L     0.3637  gm./L 

11.7 

13b 

1.87 

2.3144     ' 

0.1118     ' 

21.8 

13c 

3.04 

1.3711     " 

0.06504  " 

21.4 

13d 

7.75 

tt.5396     " 

0.02657  " 

20.6 

13e 

13.80 

0.3019     " 

0.01485  " 

20.6 

Sol.  No.  17,  after  having  been  allowed  to  dialyze  at  about 
60°  for  two  months,  was  then  subjected  to  the  same  treatment  as 
No.  13. 

TABLE  IV 


Number 
of 
Sample 

Dilution 

Ferric  Oxide 
(Fe203) 

T-       •      n  1     -j               Molar 
Ferr»c   Chloride          Ratio  of 
(FeCl3)             Fe2O3/FeCl3 

V 

17 
17a 
I7b 
17c 
17d 
17e 

2.19 
3.24 
4.44 
8.08 
8.87 

5.5385 
2.5305 
1.7102 
1.2471 
0.6833 
0.6245 

gm./L 

0.4326  gm./L 
0.1275     ' 
0.08270  " 
0.06097  " 
0.02894  " 
0.02693  " 

12.9 
20.1 
21.0 
20.7 
23.9 
23.5 

10 


In  the  last  two  a  considerable  amount  of  gel  had  formed. 

It  was  deemed  probable  that  the  gradual  precipitation,  which 
sets  in  after  the  initial  appearance  of  precipitate,  might  be  due 
to  excessive  hydrolysis  and  dialysis  near  the  walls  of  the  collodion 
bags.  If  so,  this  would  affect  the  limiting  ratio.  To  test  it,  a 
series  of  sols  was  dialyzed  in  small  unglazed  porcelain  cups.  The 
solutions  were  stirred  throughout  the  entire  period  by  a  current 
of  nitrogen  and  were  kept  at  a  temperature  of  50°-60°,  and  were 
analyzed  when  a  turbidity  became  perceptible.  The  following 
results  were  obtained : 


TABLE  V 


Number 
of 
Sample 

Dilution 

Time  of 
Dialysis 

Ferric  Oxide 
(Fe203) 

Molar 
Ferr'c  Chloride          Ratio  of 
(FeCl3)              Fe203/FeCl3 

23 
23a 
23b 
23c 

1.55 
2.14 
2.66 

7  days 
2     " 

5.4244 
3.5111 
2.5388 
2.0409 

gm.  /  L 

0.3864  gm./L 
0.1723     " 
0.1204     " 
0.09497" 

14.3 
20.7 
21.4 
21.8 

Hydrosol  No.  21,  after  three  months  dialysis,  was  divided 
into  portions  which  were  diluted  with  distilled  water  to  different 
extents,  and  these  were  dialyzed  in  porcelain  cups  at  60°  (being 
stirred  by  a  current  of  nitrogen)  until  incipience  of  precipitation 
with  the  following  results : 

TABLE  V 


Number 
of 
Sample 

Dilution 

Ferric     Oxide     Ferric   Chloride 
(Fe2O3)                   (FeCl3) 

Molar 
Ratio    of 
Fe203/FeCl3 

21 

5.867     gm./L      0.3743  gm./L 

15.9 

21a 

None 

4.9186 

0.2453     ' 

20.4 

21b 

1.33 

3.6492 

0.1829     " 

20.3 

21c 

2.00 

2.3434 

0.1160     ' 

20.6 

21d 

2.66 

1.8119 

0.09340  " 

19.7 

21e 

4.00 

1.4292                      0.06690  " 

21.6 

Following  the  incipience  of  precipitation,  gradual  flocculation 
was  observed  in  all  cases.  Analyses  of  samples  of  such  sols 
showed  that  their  molar  ratios  of  ferric  oxide  to  ferric  chloride 
gradually  increased  as  dialysis  continued  to  final  complete  precipi- 
tation. These  analyses  were  not  very  accurate  because  it  was 
impossible  to  get  the  hydrosol  entirely  free  from  precipitated 
particles  that  were  held  in  suspension.  Centrifuging  at  about 
•1,000  times  gravity  for  the  purpose  of  removing  these  suspended 
particles  frequently  resuite.l  in  the  breaking  out  of  the  entire  dis- 
persed phase  from  dispersion  in  the  form  of  a  fairly  continuous 
jelly  phase.  This  is  very  significant  in  that  it  reveals  the  jelly-like 
nature  of  this  hydrosol.  Due  to  the  inaccuracies  of  the  analyses 
the  results  are  not  reported,  but  it  is  of  interest  to  recall  that 

11 


Duclaux27  claimed  that  ferric  oxide  hydrosol  could  be  dialysed 
to  a  limiting  value  of  170  Fe2O3.l  FeQ3. 

Since  the  dispersed  phase  of  hydrosols,  prepared  as  just 
described,  migrate  to  the  cathode  when  subjected  to  the  action 
of  an  -electrical  current,  the  particles  are  naturally  said  to  be 
positively  charged  due  to  the  ferric  chloride  of  the  complex,  the 
ferric  ions  thereof  remaining  in  contact  with  the  ferric  oxide 
while  the  chloride  ions  are  located  in  the  water  phase  directly 
bathing  the  particles.  Since  like-charged  bodies  repel  one 
another,  the  electrical  charges  of  the  particles  are  supposed  to 
overcome  the  mutual  attractive  forces  of  the  particles  and  this  is 
the  commonly  accepted  explanation  for  the  stability  of  inorganic 
collodial  particles. 

According  to  this  explanation,  the  limiting  ratio  of  ferric 
chloride  to  ferric  oxide  should  increase  with  increased  concen- 
tration of  the  particles,  since  the  mutual  attractive  force  varies 
inversely  as  some  power  of  the  distance  between  the  particles. 
The  more  closely  the  particles  are  packed,  the  greater  should  be 
the  charge  required  to  keep  them  repelling  one  another. 

Examination  of  the  data  reveals  no  such  tendency.  In  fact. 
in  a  few  cases  a  concentrated  sol  showed  a  lower  limiting  ratio 
than  a  more  dilute  one.  Apparently  the  electrical  charge  is  not 
the  predominating  factor  for  the  stability.  The  fact  that  the 
limiting  ratio,  i.e.,  the  point  corresponding  to  incipience  of  pre- 
cipitation, is  always  nearly  the  same*  indicates  that  no  matter 
what  the  concentration  of  the  sol  is,  one  mole  of  ferric  chloride 
is  required  to  keep  about  21  moles  of  ferric  oxide  dispersed  in 
the  colloidal  condition.  Any  amount  of  ferric  chloride  in  excess 
of  this  ratio  might  be  regarded  as  impurity.  The  stability  of 
the  ferric  oxide  hydrosol  must  then  be  due  not  to  the  electrical 
charge  of  the  particles  but  to  the  solution  forces  (solubility)  of 
the  adsorbed  ferric  chloride.  The  high  solution  forces  of  the 
ferric  chloride  molecules  pull  the  ferric  oxide  particles  with  which 
they  are  combined  by  secondary  valence  or  "adsorption  forces" 
into  semisolution.  Upon  removal  of  the  ferric  chloride  by 
hydrolysis  the  insoluble  particles  of  ferric  oxide  having  lost  their 
"solution-link",  precipitate. 

According  to  the  "solution-link"  hypothesis  this  hydrosol 
should  be  soluble  in  any  liquid  in  which  ferric  chloride  dissolves. 
It  was  found  that  dilution  of  the  hydrosol  with  alchol  ad  infinitum 
had  no  effect.  Addition  of  ether  to  this  alcosol  did  not  precipitate 
it  either,  provided  too  large  an  excess  was  not  added. 

An  iron  oxide  hydrosol  stabilized  by  ferric  sulfate  should  be 
precipitated  by  alcohol  according  to  the  hypothesis.  Such  a  sol 

*The  slipht  deviations  from  the  vahie  of  21  may  be  ascribed  as  due  to  errors  in  the 
determinations  of  the  end  point  of  dialysis  by  means  of  the  observation  of  the  begin- 
ning of  precipitation. 

12 


was  prepared  resembling  the  Pean  de  St.  Gilles'  sol  in  appearance. 
Addition  of  alcohol  precipitated  it  instantly. 

Hydrogen  Ion  Concentration. 

Pauli  and  Matula25  attempted  to  measure  the  hydrogen  ion 
concentration  of  ferric  oxide  hydrosols.  Good  results  were  ob- 
tained by  them  when  using  sols  which  had  "aged"  for  six  months 
or  which  were  heated  for  a  few  hours  at  80°,  this  being  equivalent 
to  aging,  i.e.,  hydrolyizng  excess  ferric  chloride.  Their  measure- 
ments had  to  be  made  quickly  and  their  platinum  electrodes  were 
saturated  with  hydrogen  before  coming  in  contact  with  the  hydro- 
nrutral,  i.-e.,  a  CH+of  the  order  of  10-7. 

Measurements  of  hydrogen  ion  concentration  would  be  iM- 
possible  in  the  presence  of  ferric  ion  for  obvious  reason.  Using 
well  dialyzed  sols  in  which  the  concentration  of  ferric  ions  was 
supposedly  nil  we  found  noi  evidence  of  a  disturbing  reducing 
potential  but  could  not  get  what  is  considered  to  be  an  absolutely 
satisfactory  equilibrium  reading  due  to  the  deposition  of  ferric 
oxide  gel  upon  the  plantinizecl  electrode.  However,  taking  the 
mean  of  a  series  of  readings  which  were  not  widely  divergent,  a 
CH+  of  10~4-9  was  indicated.  This  was  the  same  for  a  series  of 
our  "pure"  hydrosols  of  varying  concentration  and  consequently 
the  hydrogen  ion  concentration  does  not  appear  to  depend  upon 
the  concentration  of  the  dispersed  phase,  at  least  over  the  range 
which  we  studied.  We  would  say  that  our  "pure"  ferric  oxide 
hydrosols  showed  a  CH+  =  10~5. 

An  acid  reaction  is  to  be  expected  since  upon  dialysis  of 
sols  from  which  the  free  ferric  chloride  has  been  removed,  only 
hydrogen  and  chloride  ions  are  found  in  the  clifFusate  across  the 
collodion  membrane.  Consequently  the  ferric  chloride  of  the 
dispersed  phase  is  in  equilibrium  with  the  ions  of  hydrochloric 
acid  in  the  dispersion  medium,  which  in  the  case  of  our  "pure" 
sols  is  of  the  order  of  M  / 100, 000  (or  less,  since  part  of  the 
acidity  may  be  due  to  carbonic  acid.) 

Behavior  upon  Freezing. 

As  previously  mentioned,  freezing  point  depression  was  tried 
as  a  qauntitative  measure  for  following  purification  but,  it  was 
found  that  a  well  dialysed  sol  gives  a  depression  of  the  freezing- 
point  within  the  range  of  experimental  error  of  measuring  the 
same  by  means  of  the  Beckmann  thermometer.  Consequently 
any  molecular  weight  figures  based  upon  such  determinations,  as 
given  by  several  investigators  28,  29,  are  valueless  as  has  been 
previously  suggested,  20-  30 

Pean  de  St.  Gilles  was  inclined  to  regard  his  hydrosols  as 
true  solutions  because  they  froze  in  a  "normal"  manner.  In  1889, 

13 


Ljubawin31  froze  among  otlier  colloidal  substances,  ferric  oxide 
hydrosol  and  found  that  on  continued  cooling  particles  of  ferric 
oxide  concentrated  in  the  center  while  the  periferous  layers  of  the 
ice  became  colorless.  Upon  melting,  all  of  the  iron  oxide  par- 
ticles redispersed.  Lottermoser32  found  that  only  sols  which  are 
deficient  in  electrolyte  will  precipitate  out  on  freezing.  Sols  rich 
in  electrolyte  are  not  affected  even  on  continued  freezing. 

Our  experiments  showed  that  when  a  pure  ferric  oxide 
hydrosol  is  only  partially  frozen,  ice  crystals  are  formed  which, 
upon  melting,  leave  the  sol  as  homogeneous  as  it  was  before  the 
operation.  But  when  cooling  is  continued  until  freezing  is  com- 
plete, the  sol  on  melting  will  undergo  some  precipitation.  The 
longer  the  sol  has  been  cooled,  the  greater  will  be  the  amount  of 
gel  formed.  The  precipitate  is  in  the  form  of  short,  amorphous, 
shiny,  needle-like  particles.  When  the  sol,  placed  in  a  test-tube, 
is  allowed  to  remain  in  contact  with  the  ice-salt  mixture  for  some 
length  of  time,  the  entire  solution  turns  to  a  dark  red  solid  mass. 
Upon  continued  cooling,  separation  of  the  water  begins  as  a 
layer  of  colorless  ice  near  the  walls  of  the  test-tube,  and  such 
layers  continue  inward  until  in  the  center  there  are  deposited 
the  red  brown  particles  above  described.  These  particles  are  ar- 
ranged in  a  string-like  formation  throughout  the  height  of  the 
tube.  Upon  melting  the  ice  they  do  not  redisperse.  The  water 
obtained  from  melting  the  ice  shows  a  barely  perceptible  test  for 
chloride  ion  and  none  for  ferric  ion.  The  gel  particles  are  prac- 
tically insoluble  in  dilute  nitric  acid,  but  readily  soluble  in  con- 
centrated acid.  Analysis  of  them  showed  that  about  80  percent 
of  the  ferric  chloride  of  the  original  hydrosol  particles  was  re- 
tained in  this  gel.  This  behavior  is  quite  different  from  that  of 
Bredig  gold  hydrosols  upon  freezing,  since  Beans  and  Beaver33 
find  that  all  of  the  stabilizing  electrolyte  is  removed  from  the  gold 
particles  through  the  congelation. 

These  observations  strengthen  the  conclusion  reached  from 
the  limiting  ratio  of  the  hydrosol  that  the  stability  of  those 
hydrosols  is  due  to  the  solution  forces  of  the  adsorbed  ferric 
chloride  rather  than  to  the  electrical  charge  of  the  particles. 

The  ferric  chloride  in  the  congelation  gel  from  the  pure  sols 
must  be  dispersed  throughout  the  compact  slid  mass,  for  although 
there  is  sufficient  ferric  chloride  present  to  redisperse  the  par- 
ticles, at  least  partially,  such  does  not  take  place,  but  when  an 
impure  sol  is  frozen,  i.e.,  one  to  which  some  excess  ferric  chloride 
had  been  added,  the  gel  particles  redisperse  upon  melting.  In 
the  latter  case  the  gel  cannot  be  so  massive  as  in  the  case  of  the 
gel  from  a  pure  sol  and  consequently  the  pull  of  the  water  on 
the  adsorbed  ferric  chloride  is  able  to  disintegrate  the  particles 
and  redisperse  them.  The  appearance,  upon  freezing,  of  an  im- 
pure sol  is  different  from  that  of  a  pure  sol.  The  preliminary  ice 

14 


mush  has  a  lighter  color  and  upon  complete  congelation  the  gel 
particles  deposited  in  the  center  are  somewhat  larger  in  appear- 
^ance  and  less  distinct  individually. 

In  a  recent  article,  Gutbier  and  Flury34  reported  that  in  the 
case  of  selenium  oxide  sols,  the  extend  of  the  reversibility  after 
freezing  depends  upon  the  amount  of  peptizing  electrolyte 
originally  present.  A  sufficiently  well  dialysed  selenium  oxide 
sol  is  entirely  irreversible  after  freezing;  while  the  deposit  from 
an  impure  sol  readily  redissolves  upon  melting. 

Relationship  Between  Graham's  Hydrosol  and  the  So^Called 
"Metairon"  Hydrosol  of  Pean  de  St.  Gilles.  Water  of 

Hydration. 

The  hydrosol  that  Pean  de  St.  Gilles  prepared,  by  heating  and 
boiling  solutions  of  the  acetate,  differed  slightly  in  properties  from 
Graham's  in  that  it  was  not  so  clear  and  that  a  precipitate  formed 
on  continued  heating  which  was  insoluble  in  concentrated  acids 
but  soluble  in  dilute  acids  and  water.  Graham,  in  analogy  to  the 
two  modifications  of  tin  oxide  sol,  called  it  the  "metairon"  oxide 
hydrosol.  This  appellation  is  now  common  and  it  is  generally 
admitted  that  a  study  of  this  modification  is  greatly  needed. 

In  the  course  of  this  investigation,  it  was  found  that  this 
conception  of  two  modifications  of  ferric  oxide  hydrosol  is  not 
justifiable,  the  main  difference  between  the  two  being  water  of 
hydration  of  the  particles. 

When  ferric  chloride  or  hydrochloric  acid  is  added  to  the 
ordinary  Graham  hydrosol,  an  ochre  colored  precipitate  is  formed 
which  dries  on  a  porous  plate  to  a  chocolate  colored  mass.  This 
dry  mass,  as  well  as  the  original  wet  precipitate,  is  readily  soluble 
in  water,  giving  rise  to  a  hydrosol  identical  in  all  its  properties 
to  the  "metairon"  modification.  On  redispersion  of  the  powder 
an  water,  the  resulting  sol  is  slightly  turbid,  because  during  the 
precipitation  some  dehydration  of  the  colloidal  particles  took 
place.  The  more  thoroughly  the  precipitate  is  dried  before  its 
redispersion,  the  more  turbid  is  the  solution  formed.  After  dry- 
ing in  an  oven  at  110°,  it  forms  an  unstable  dispersion  of  yellow 
particles  in  water  which  settle  out  after  standing  a  few  days. 
In  alcohol,  with  which  Graham's  hydrosol  is  miscible  in  all  pro- 
portions, this  wet  precipitate  forms  a  coarse  dispersion  of  yellow 
particles  similar  to  that  formed  by  dispersing  the  over-dried 
precipitate  in  water. 

When  precipitated  ferric  hydroxide  washed  free  from 
ammonium  salts  is  treated  with  a  solution  containing  hydrochloric 
acid  or  ferric  chloride  and  is  allowed  to  stand  for  a  few  months, 
the  hydrosol  formed  is  more  turbid  than  the  "metairon"  sol. 
This,  again,  may  be  attributed  to  a  lower  hydration  or  to  a  larger 
size  of  the  particles. 

15 


But  if  this  were  an  effect  of  the  size  of  the  particles,  we 
should  expect  the  limiting  values  of  ferric  chloride  to  be  higher 
than  that  established  above,  that  is,  more  electrolyte  would  be 
required  to  keep  the  larger  particles  dispersed.  The  following 
experiment  was  made  to  decide  this  question. 

A  hydrosol  was  prepared  by  addition  of  ammonium  hyd- 
roxide to  ferric  chloride  solution  until  the  resultant  precipitate 
no  longer  readily  peptized.  Ferric  chloride  was  then  added  to 
precipitate  the  dispersed  phase  and  the  gel  so  obtained  was  dried 
on  a  porous  plate.  This  residue  was  then  dispersed  in  water 
resulting  in  a  sol  slightly  turbid  to  reflected  light.  This  was 
dialyzed  in  collodion  sacks  at  room  temperature  for  three  months 
when  it  showed  upon  analysis  a  molar  ratio  of  Fe2O3/FeCl3 
=18.0.  A  portion  of  this  dialyzed  sol  was  then  dialyzed  in  a 
porcelain  cup  at  about  60°  with  continuous  stirring  by  a  current 
of  nitrogen.  It  was  analyzed  (a)  after  five  days  of  dialysis  and 
then  again  (b)  after  nine  days.  In  (a)  incipience  of  precipita- 
tion was  not  evident  while  in  (b)  a  decided  precipitation  had 
started.  The  ratios  of  Fe2O3/FeCl3  were  (a)  19.7  and  (b) 
25.9.  Another  hydrosol  prepared  by  peptizing  ferric  hydroxide 
gel  with  a  small  amount  of  hydrochloric  acid  was  dialyzed  in 
porcelain  as  described  above.  This  initially  turbid  sol  increased 
in  turbidity  as  dialysis  was  continued  so  that  the  beginning  of 
precipitation  could  not  be  determined  accurately.  When  the  end 
point  was  presumed  to  have  been  reached,  it  showed  upon  anal- 
ysis a  ratio  of  Fe2O3/FeCl3=22.6. 

It  is  thus  seen  that  the  limiting  value  for  these  "meta-iron 
oxide,"  or  Pean  de  St.  Gilles'  sols  is  of  the  same  magnitude 
as  that  of  the  Graham  sol.  This  would  indicate  that  the  turbidity 
of  this  sol  is  due  to  dehydration  rather  than  to  the  presence  of 
larger  ferric  oxide  aggregates. 

It  has  been  suggested  before  that  the  difference  between 
the  two  types  of  ferric  oxide  sols  is  one  of  hydration35.  The 
Graham  sol  was  suggested  to  be  FeoO3.3HoO  and  the  Pean  de 
St.  Gilles,  Fe2O3.H2O. 

Since,  according  to  Einstein36,  the  viscosity  of  a  colloidal  dis- 
persion is  expressed  by  the  formula  N*=N  (1-j-kf),  where  N* 
=viscosity  of  the  system,  Af=viscosity  of  the  dispersion  medium, 
and  f=the  ratio  of  total  volume  of  the  dispersed  phase  over  the 
total  volume  of  the  system,  which  means  as  Wo.  Ostwald37  and 
others  have  shown  that  the  viscosity  of  a  colloidal  solution  in- 
creases with  the  amount  of  the  dipersion  medium  taken  up  by 
the  dispersed  phase,  the  Pean  de  St.  Gilles  sol  should  have  a 
lower  viscosity  than  the  Graham  sol  of  the  same  concentration  if 
our  supposition  is  correct. 

To  determine  this,  lOcc.  portions  of  a  Graham  hydrosol  were 
treated  with  varying  amounts  of  2N  ferric  chloride  until  the  tur- 

16 


bidity  which  first  appeared  gradually  changed  to  coarse  bright 
yellow  dispersion  and  the  viscosities  were  measured. 

The  measurements  were  made  by  means  of  an  Ostwald  vis- 
cosimeter,  in  a  constant  temperature  bath  at  25  ±0.1°.  The 
following  tabluation  shows  the  results.  The  figures  signify  the 
time  in  seconds  for  outflow : 

(1)    Sol  No.   15          10      cc.  81   sec.  Distilled  Water  71  sec. 


(2) 
(3) 
(4) 
(5) 
(6) 
(7) 


+     0.1  cc.  FeCl3       73   sec.   10  cc.  +2  cc.FeC!3          -80 


4-  0.2  "  73 

+  0.3  "  74 

4-  0.5  '  "            75 

+  1.0  "  "            78 

+  2.0  "  "           99 


10  cc.  2N  FeCl3  —137 


Number  1  was  perfectly  clear,  numbers  2,  3,  4,  and  5  were 
clear  to  transmitted,  but  turbid  to  reflected  light.  They  resembled 
the  Pean  de  St.  Gill-es  sol  in  all  respects.  The  turbidity  gradu- 
aly  increased  with  increasing  amounts  of  ferric  chloride.  Number 
6  was  decidedly  brown  while  No.  7  was  yellow.  After  standing 
for  three  hours,  the  viscosities  were  again  measured  and  found 
to  be  unchanged. 

From  these  results  it  is  seen  that  the  addition  of  ferric 
chloride  first  decreases  the  viscosity  and  then  increases  it.  The 
decrease  in  viscosity  indicates  a  diminution  in  size  of  the  parti- 
cles which  could  only  have  been  caused  by  a  loss  of  water  of 
hydration  by  the  dispersed  phase,  i.e.,  dehydration.  The  increase 
in  viscosity  observed  upon  the  addition  of  larger  amounts  of 
ferric  chloride  is  due  to  the  coalescence  of  the  particles  prelim- 
inary to  precipitation. 

As  to  the  mechanism  of  the  dehydration  of  the  sol  by  ferric 
chloride,  we  can  only  venture  to  say  that  it  is  possibly  due  to 
the  high  hydration  of  the  electrolyte  added,  thus  causing  a  partial 
dehydration  of  the  dispersed  Fe2O3 — FeCl3 — H2O  phase,  or  in 
view  of  the  fact  that  this  sol  will  migrate  in  an  electrical  fi-eld 
showing  that  the  adsorbed  and  peptizing  electrolyte  is  ionized, 
even  though  the  degree  be  extremely  small,  then  the  Donnan 
effect  of  the  added  ferric  chloride  would  result  in  a  decrease  in 
swelling  (hydration)  of  the  dispersed  phase  as  in  the  case  of 
the  addition  of  hydrochloric  acid  or  a  neutral  salt  to  gelatin  jelly 
swollen  in  a  solution  of  hydrochloric  acid.  Both  suggested  mech- 
anisms may  operate  at  the  same  time. 

NEGATIVE  IRON   OXIDE  HYDROSOL 

Linder  and  Picton  in  1892  and  Coehn  in  189838  have  shown 
that  ferric  oxide  hydrosol  is  charged  positively. 

H.  W.  Fisher39  prepared  a  negatively  charged  ferric  oxide 
hydrosol  by  runnings  two  thirds  normal  ferric  chloride  solution 
into  five  normal  sodium  hydroxide  solutions  to  which  a  consid- 
erable amount  of  glycerol  had  been  added.  It  is  by  no  means 

17 


certain  that  a  negative  iron  oxide  hydrosol  was  thus  prepared, 
and  Fisher  himself  admits  the  possibility  of  the  formation  of  a 
compound  with  the  glycerol  which  would  account  for  its  anodic 
migration,  analogous  to  the  action  of  tartrate  in  Fehling's  solution. 

Powis40  prepared  a  negative  sol  by  slow  addition  of  lOOcc. 
of  .01V  ferric  chloride  to  loOcc.  .OlV  sodium  hydroxide  solution 
with  constant  stirring.  The  sol  was  reddish  brown,  clear,  and 
remained  for  several  weeks  without  precipitating.  On  dialysis, 
it  precipitated  in  a  few  hours.  He  prepared  a  similar  sol  by 
using  a  positive  sol  instead  of  the  ferric  chloride  used  above  and 
concluded  from  this  experiment  that  the  hydrosol  ion  is  adsorbed 
and  that  the  negative  charge  is  due  to  it. 

H.  B.  Kruyt  and  J.  Van  de  Speck41  find  that  upon  adding 
from  1.55  to  2.8  millimoles  of  sodium  hydroxide  to  a  ferric 
oxide  hydrosol  containing  0.75  gm.  of  ferric  oxide  and  0.064  gm. 
of  chlorine  per  kilogram  and  allowing  the  mixture  to  stand  for 
three  hours,  complete  precipitation  takes  place  and  in  some  cases 
immediately.  The  sol  is  not  precipitated  when  sodium  hydroxide 
of  the  concentration  of  3.99  to  27.9  millimoles  is  added,  although 
it  becomes  turbid.  When  more  than  30.3  millimoles  is  added, 
complete  precipitation  again  ensues.  They  call  this  zone  of  no 
precipitation  the  "tolerance"  zone  of  ferric  oxid-e  sol  and  con- 
sider the  substance  within  that  zone  to  consist  of  a  negatively 
charged  ferric  oxide  hydrosol. 

In  this  laboratory,  Powis's  experiments  were  duplicated, 
using  ferric  chloride.  Clear  reddish  brown  solutions  were  ob- 
tained which  were  precipitated  by  the  addition  of  sodium  sul- 
fate.  However,  none  of  the  sols  prepared  in  the  course  of  these 
experiments  was  stable.  A  gradual  settling  began  after  a  couple 
of  days  and  all  the  dispersed  phase  settled  out  within  a  week 
or  two. 

With  well  dialyzed  positive  sols,  the  "negative  sol"  was 
-obtained  only  when  extremely  high  dilutions  were  used.  The 
dispersions  formed  were  stable  for  only  a  few  hours  and  in  no 
case  remained  dispersed  longer  than  a  day.  The  addition  of 
sodium  sulfate  caused  immediate  precipitation  of  ferric  oxide. 

Using  .01V  ferric  chloride  and  various  concentrations  of 
sodium  hydroxide,  it  was  found  that  concentrations  above  .OlV 
.sodium  hydroxide  resulted  in  the  immediate  formation  of  sus- 
pensions which  settled  out  within  a  few  hours.  Increasing  the 
concentrations  of  ferric  chloride  likewise  resulted  in  suspensions 
which  s-ettled  out  within  a  short  time. 

These  dispersions  could  be  thrown  down  by  centrifuging 
immediately  after  their  preparation  at  1,000  times  gravity  only 
when  they  were  more  or  less  concentrated.  For  instance,  when 
a  considerable  amount  (lOOcc.)  of  .01  N  ferric  chloride  was  added 

18 


to  .01  A7"  sodium  hydroxide,  ferric  hydroxide  was  thrown  out  on 
centrifuging  quite  easily;  but  when  smaller  amounts  of  ferric 
chloride  (up  to  50cc)  were  added,  centrifuging  had  no  effect  on 
them.  On  standing,  for  a  day  at  most,  they  would  begin  to  settle 
out.  These  samples  from  which  ferric  oxide  gel  could  not  be 
thrown  down  by  centrifuging  immediately  after  their  preparation, 
when  allowed  to  stand  for  a  few  hours  or  a  day,  depending  upon 
the  dilution,  were  easily  thrown  out  on  centrifuging. 

Powis  believes  these  solutions  to  consists  of  a  negative  ferric 
oxide  hyrosol  due  to  the  adsorption  of  hydroxyl  ions  by  the  ferric 
oxide  particles.  However,  it  is  conceivable  that  it  may  be  a 
negative  hydrosol  stabilized  by  a  ferrate  ion  or  that  it  may  be  a 
negative  suspension. 

Since  none  of  the  conditions  considered  necessary  for  the 
preparation  of  a  ferrate  is  satisfied  in  these  experiments,  a  fer- 
rate is  presumably  not  formed,  although  its  formation  would  fit 
the  behavior  of  this  sol  remarkably  well. 

It  is  our  opinion  that  the  solution  formed  in  these  cases  are 
negative  suspensions.  There  is  no  reason  why  settling  of  ferric 
oxide  should  begin  when  the  solutions  are  allowed  to  remain  in 
stoppered  "Non-Sol"  bottles. 

SUMMARY 

1.  It  has   been   shown   that   coloidal    ferric   oxide   remains 
stable  as  long  as  the  particles  contain  one  mole  of  ferric  chloride 
to  approximately  21  moles  of  ferric  oxide.     Beyond  this  point 
gradual  precipitation  begins  with  the  formation  of  hydrosols  of 
lower  ferric  chloride  and  lower  ferric  oxide  content. 

2.  Evidence  has  been  offered  o  show  that  ferric  oxide  hydro- 
sol is  not  merely  a  complex  consisting  of  ferric  oxide  and  ferric 
chloride  but  that  water  is  a  third  essential  part  of  the  colloidal 
particle ;  and  that  the  stability  of  ferric  oxide  hydrosol,  stabil- 
ized by  ferric  chloride,  is  due  to  the  solution  forces  of  the  ab- 
sorbed ferric  chloride  in  the  dispersed  medium  rather  than  to  the 
mutual  repulsive  forces  of  the  particles  presumed  to  reside  in 
their  electrical  charges  of  like  sign. 

3.  The  so-called  "metairon"  sol  of  Pean  St.  Gilles  has  been 
shown  to  be  merely  a  les  hydrated  form  of  Graham's  sol. 

4.  The   hydrogen    ion    concentration   of    pure    ferric   oxide 
hydrosol  has  been  shown  to  be  approximately  10~4-9. 

5.  Freezing  of  pure  ferric  oxide  hydrosols  causes  irreversible 
coagulation,  a  small  part  of  the  peptizing  ferric  chloride  being 
split  off  and  the  greater  part  remaining  in  the  gel. 

6.  The  reversibility  of  the  sign  of  the  charge  of  ferric  oxide 

19 


hydrosol  to  form  negative  sols  has  been  investigated  and  -evidence 
offered  which  shows  that  if  a  negative  sol  is  actually  formed  it 
it  exceedingly  unstable  and  probably  is  merely  a  suspension  of 
ferric  oxide  gel. 


BIBLIOGRAHY 

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VITA 

Alexander  Frieden  was  born  October  5,  1895  and  attended 
the  public  schools  of  Norfolk,  Virginia.  He  entered  the  Uni- 
versity of  Virginia  in  September,  1915  and  graduated  in  June, 
1919,  receiving  the  degrees  of  B.S.  and  M.S.  Since  September, 
1919,  he  has  been  a  graduate  student  in  Chemistry  under  the  Fac- 
ulty of  Pure  Science,  Columbia  University.  In  Jun-e,  1920,  he 
received  the  degree  of  Master  of  Arts  from  Columbia  University. 

During  the  war  he  was  a  member  of  the  Chemical  War- 
fare Service,  U.  S.  A.  Since  September  1920  he  has  occupied 
the  position  of  Assistant  in  Chemistry  at  Columbia  University. 


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8  1935 


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Binder 
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